1. Field of the Invention
The present invention relates to a method and an apparatus for cutting and grinding a single crystal SiC for use in a hard electronics.
2. Description of the Prior Art
The hard electronics generically designates a strong electronics which uses a wide gap semiconductor such as SiC or diamond having a value of physical property above that of silicon as a base and can meet a hard specification over this limit. A band gap of SiC or diamond which is dealt in the hard electronics ranges from 2.5 to 6 eV, as compared with 1.1 eV of Silicon.
The history of the semiconductor started from germanium and shifted to silicon having a larger band gap. The largeness in the band gap is associated with that in chemical binding power between atoms constituting a matter, and not only is a material very hard, but a value of physical property required for the hard electronics such as a dielectric breakdown electric field, a carrier saturation drift velocity, a thermal conductivity and others is far superior to that of silicon. For example, there is a Johnson index to a high-speed and large-output device as one performance index of the hard electronics and, assuming that the index of silicon is 1, the index of the semiconductor of the hard electronics decuples or centuples that value as shown in FIG. 1.
Therefore, the hard electronics is expected as a substitution for the conventional silicon semiconductor in the fields of the energy electronics represented by a power device, the information electronics in which milli-meter wave/microwave communication is mainly dealt, the extreme environment electronics such as nuclear energy, geothermal sources, space and the like.
In the hard electronics, the study of an SiC power device is most advanced. However, even in SiC with which studies for realizing devices are most advanced, the conventional silicon processing technique can not be directly applied for realizing the elemental device because SiC has strong chemical binding power and is a hard material.
That is, in order to manufacture a device from an ingot of a single crystal SiC, the ingot must be cut out in a tabular form and its surface must be flatly finished as in the prior art. However, when applying conventional silicon cutting means to cutoff of the single crystal SiC, the finishing speed is slow and a step called a saw mark tends to be produced on the cut surface because the single crystal SiC is a hard and chemically stable material. When such a step is once produced, a very long time is required for mechanically grinding to obtain a flat surface because the single crystal SiC is a hard and chemically stable material, thereby largely reducing the productivity of the hard electronics material.
Further, in the conventional silicon, the roughness of a cut surface obtained by the cutting means is planed by polishing by another device using chemical etching after cutoff. However, the chemical etching applied to a conventional silicon material is hard to be applied to the single crystal SiC which is a chemically stable material for this planation.
The present invention is intended to solve the above-described problems. That is, it is an object of the present invention to provide a method and an apparatus for cutting and grinding a single crystal SiC, by which an ingot of the single crystal SiC can be efficiently cut out in a tabular form and its cut surface can be finished to be as flat as a mirror surface.
As grinding means for realizing highly-efficient/superfine specular grinding which is impossible in the conventional polishing technique, an electrolytic in-process dressing grinding method (which will be referred to as an ELID grinding method hereinafter has been developed by the present applicant. According to this ELID grinding method, a conductive bonding portion of a metal bond grind stone is dissolved by the electrolytic dressing and ground while performing truing. By this grinding method, use of the metal bond grind stone having fine abrasives enables excellent grinding which is efficient to the hard material, and the high streamline/ultrasophistication can be intended. The present invention can take advantages of the ELID grinding method and utilizes this method to the grinding and the cutoff of the single crystal SiC.
That is, according to the present invention, there can be provided a method for cutting and grinding a single crystal SiC, wherein a metal bond grind stone (10) is applied to positive potential while an electrode opposed to this metal bond grind stone is applied to negative potential; a conductive liquid (15) is supplied between the metal bond grind stone and the electrode; the surface of the metal bond grind stone is subjected to the electrolytic dressing by applying a direct-current pulse voltage between the metal bond grind stone and the electrode while an ingot (1) of a single crystal SiC is cut out by using the metal bond grind stone (10); and the cut surface is then subjected to grinding by using the metal bond grind stone.
According to the method of the present invention, although the cutting and the grinding can be performed using separate grind stones or apparatuses, when the surface of the metal bond grind stone (10) is subjected to the electrolytic dressing while cutting the ingot (1) of the single crystal SiC by using the metal bond grind stone and the metal bond grind stone is then used for the grinding of the cut surface, even the ingot of the hard single crystal SiC can be efficiently cut out by using the abrasives trued by the electrolytic dressing. Further, since the surface of the metal bond grind stone can be precisely trued by the electrolytic dressing, the cut surface can be finished to be as flat as a mirror surface by using the fine abrasives.
According to a preferred mode for embodying the present invention, the metal bond grind stone consist of a cast iron based metal binding material and diamond abrasives having particle sizes different at a flat plate portion (10a) and a tapered portion (10b), and the ingot (1) of the single crystal SiC can be cut off by the tapered portion (10b) so that the cut surface can be subjected to the grinding by the flat plate portion (10a).
By this method, since the both surfaces of the tapered portion (10b) can obliquely cut into the ingot (1) of the single crystal SiC by only moving the metal bond grind stone (10) in a direction orthogonal to an shaft center, the efficient cutoff is possible. Furthermore, since the flat plate portion (10a) is provided to the inner side, the cut surface can be finished on a flat surface orthogonal to the shaft center of the grind stone.
Moreover, it is preferable that the flat plate portion (10a) and the tapered portion (10b) of the metal bond grind stone (10) are composed of diamond abrasives having different particle size""s and an iron cast based metal binding material.
With this structure, when the particle size in the flat plate portion (10a) is minimized and that in the tapered portion (10b) is roughened for example, the efficiency at the time of cutoff is improved and the finishing precision of the cut surface can be enhanced.
In addition, according to the present invention, there is provided an apparatus for cutting and grinding a single crystal SiC comprising: a metal bond grind stone (10) constituted by a flat plate portion (10a) rotating around a shaft center and a tapered portion (10b) which is provided to the outside of the flat plate portion and formed in such a manner that its outer side is gradually thinned; an electrode (13) opposed to the metal bond grind stone with a gap therebetween; voltage applying means (12) for applying a direct-current pulse voltage between the metal bond grind stone as an anode and the electrode as an cathod; conductive liquid supplying means (14) for supplying a conductive liquid (15) between the metal bond grind stone and the electrode; and grind stone moving means (16) for moving the metal bond grind stone in a direction orthogonal to the shaft center, thereby cutting the ingot (1) of the single crystal SiC by using the tapered portion (10b) of the metal bond grind stone to then subject the cut surface to the grinding using the flat plate portion (10a).
With this structure according to the present invention, when the electrolytic dressing is applied to the taper portion (10b) of the metal bond grind stone, even the ingot of the hard single crystal SiC can be efficiently cut off by using the abrasives smoothed by the electrolytic dressing. Additionally, since the surface can be precisely trued by performing the electrolytic dressing on the flat plate portion (10a) of the metal bond grind stone, the cut surface can be finished into a flat surface orthogonal to the shaft center of the grind stone and this surface can be finished to be as flat as the mirror surface by using the fine abrasives.